Interstellar: the truth about wormholes

Christopher Nolan's new film, Interstellar, sees astronauts leaping across
galaxies via a 'wormhole'. Could it one day be possible?

The universe wasn't designed to make life easy for science-fiction writers. Nowhere is this more obvious than if you are trying to write a grand space opera.

You know the sort of thing: a dashing space pirate, a rogue with a heart of gold, fights a reluctant rebellion against a great galactic empire; he or she is dragged from a comfortable life smuggling petty contraband and forced to leap from star system to star system, one step ahead of the bad guys, before eventually winning the day and getting the girl/boy/small furry creature from Alpha Centauri.

Unfortunately, it's nonsense. The rules of the universe - the laws of physics - aren't perfectly known, but the one that is written in stone is: you can't travel faster than light. You just can't. And while light travels pretty fast - 186,000 miles per second - that's still a crawl, when it comes to the unhelpfully enormous distances between stars.

Our nearest star is Proxima Centauri, part of the aforementioned Alpha Centauri. At lightspeed, it is an intimidating 4.24 years' travel away, a long trek even for the most dashing of space pirates. And that's the nearest; obviously, most stars are a great deal farther than that.

Science fiction authors have to come up with some sort of way around this problem. Star Warswaved its hands a bit and said the Millennium Falcon had a "hyperdrive", without ever doing anything so tiresome as explaining what that actually was;Star Trek's "warp drive" is supposed to bend space itself, so that the ship doesn't, technically, go faster than light. But a much-anticipated film, released next month, is trying something else: wormholes.

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Directed by Christopher Nolan, the man behind The Dark Knight trilogy and the intellectually complex blockbuster Inception,Interstellar has been made hand-in-hand with one of astrophysics' most acclaimed scientists, 74-year-old emeritus professor Kip Thorne.

One of the leading experts on the implications of Albert Einstein's theory of general relativity,Thorne, a good friend of Stephen Hawking, revolutionised physics's attitude to the nature of space and time when he proved in a seminal 1988 research paper that it was possible for wormholes to exist.

And since Interstellar - which stars Matthew McConaughey and Anne Hathaway as astronauts hunting for a new planet that can sustain human life - is based on this breakthrough, it means the film promises to be the most scientifically plausible big-budget sci-fi blockbuster ever made.

What's more, the film's characters will not simply travel between stars in a scientifically acceptable way, they will also be able to travel in time without attracting brickbats from pedantic astrophysicists, for Thorne believes that particular feat - an obsession of science fiction writers ever since HG Wells published The Time Machine in 1895 - is also possible.

In an interview last year, Thorne said: "Whether you can go back in time is held in the grip of the law of quantum gravity. We are several decades away from a definitive understanding, 20 or 30 years, but it could be sooner than that."

Thorne is an eccentric figure - a sandal-shod, Hawaiian-shirt-clad, hippyish California type, who famously bet Hawking a year's subscription to Penthouse magazine that there was a black hole in the constellation Cygnus. (He won.) But such are his achievements, that if he says time travel is possible, it's worth taking him seriously.

To understand the problem of lightspeed, and how wormholes could "solve" it and open up the possibility of time travel, we need to know a little about the history of physics.

In the 17th century, Sir Isaac Newton expounded the laws of gravity and motion. The gravitational attraction between two objects, he said, was determined by their mass and the square of the distance between them, so an object twice as far away is pulled one-quarter as hard. Objects will keep moving through space at the same speed and direction unless some sort of force changes them. Newton's calculations were so accurate that they lasted unchallenged for more than 200 years, and are still precise enough for Nasa to use them to send space probes to distant planets. But they are wrong.

They're wrong, at heart, because Newton treated "time", "space" and "gravity" as three distinct things.That's understandable, since that's how we perceive them. But in the late 19th century, it became increasingly obvious that something didn't work. In 1865, the British physicist James Clerk Maxwell showed that all light - in fact all radiation, from radio waves to gamma rays - travels like waves through space at the same constant speed.

And then two American physicists, Albert Michelson and Edward Morley, showed that the speed of light was the same when they measured it at right angles to how the Earth was travelling, or when the Earth was heading straight into it.

Imagine how odd that is. It's as though you are on a boat, heading out into the waves. The waves hit your boat once every second. So you turn your boat to the side and travel along the line of the surf. You're no longer moving into the waves, so you expect them to hit you less often - but they don't! They hit you at exactly the same rate. Something strange is going on.

Anne Hathaway and Matthew McConaughey in Interstellar

This was where Albert Einstein came in. He realised that you could remove the problem if you dropped the idea that time is absolute. Lightspeed is the only constant. Light moves at the same speed for everyone, no matter how fast they are moving or in what direction; instead of lightspeed changing, time itself slows down; the faster you move, the slower time passes.

"If I travel to a distant star at near-light speeds, I wouldn't experience very much time passing," explains Andrew Pontzen, a cosmologist at University College London. "To an observer on Earth it would seem as though decades had passed."

This is the heart of Einstein's theory of relativity, and it has other, equally baffling implications. For one thing, it means that energy and mass are the same thing; the famous equation E=mc2 says that energy is equal to mass times the speed of light squared. And time and space are two aspects of a single thing called spacetime, and spacetime isn't a constant, steady thing, but is warped by gravity.The usual metaphor is that spacetime is a stretched sheet; objects, such as planets and stars, rest on that sheet, pulling it down, so all the objects on the sheet tend to roll towards one another.

Those facts are enough to understand, broadly, why nothing can go faster than light. As something goes faster, it gets more energy. As something gets more energy, according to Einstein, it gets more mass. This isn't noticeable, except with incredibly sensitive measurements, at the piddling speeds we humans move around at - but as you get close to lightspeed, it becomes rapidly bigger. And something moving at lightspeed itself would have infinite mass, and therefore need infinite energy. For obvious reasons, that can't happen. The speed of light is an absolute limit.

But wormholes may offer a plausible way around it, in a way that physics allows, "depending", says Pontzen, "on what you mean by 'plausible'".

"It's really more of an engineering problem than a physics one," says DrTom Whyntie, a physicist at Cern.

Pontzen agrees: "The idea has been put forward by respectable physicists, and is based on physics that we understand very well." Remember that spacetime is like a sheet. Heavy objects distort that sheet. The heavier they are, the more they distort the sheet. But eventually, if they are heavy enough, their gravity is so enormous that all the matter in it is condensed into an infinitely small volume. And when that happens, it distorts the spacetime sheet an infi-nite amount: or, if you like, tears it.

These holes in spacetime are called black holes. In the Eighties it was suggested that it would be possible for two or more of them to be connected: that it would be possible to travel from one place in the universe to another via these holes in space, in what are called "Einstein-Rosen bridges", or more commonly, "wormholes".

In an interview with Discover magazine,Thorne described it like this: "It's sort of like if a worm drilled a hole through an apple from one side to the other. If you were an ant and you lived on the surface of the apple, there could be two routes to get from one side of the apple to the other. One is around the outside, on the surface, which we can think of as being like our universe's gently curved space; the other is down the wormhole."

If you go through the wormhole, you can travel at permissible, slower-than-light speed, but still end up somewhere faster than light would, because you've taken a shortcut.

"Thorne is the world expert on this stuff," says Pontzen. "So if any film is going to give an accurate portrayal of speculative-but-not-impossible physics, it's going to be this one." Thorne has previous: he advised the cosmologist Carl Sagan on his novel Contact, later made into a film with Jodie Foster, suggesting wormholes as the most feasible way of sending a human over interstellar distances. In an interview with The Guardian last year, Thorne said Contact and 2001: A Space Odyssey are the two films he rates from a science point of view, because both Sagan and Arthur C Clarke have physics training.

Thorne and Hawking theorise that tiny wormholes appear and disappear all the time, in the "quantum foam", the tiniest scale of the universe, far, far smaller than atoms. But, says Thorne, it might be possible to make a large, stable one which humans could travel through. And indeed it might be, says Pontzen. "But it relies on some very speculative aspects, so there are huge engineering challenges involved."

First, the entrance to your wormhole needs to be made of some seriously weird stuff. Specifically, to create a wormhole that doesn't pinch closed immediately, the material needs to have a huge amount of "negative energy": it needs to contain, in essence, less than nothing.

Negative energy, also known as the Casimir effect, is possible, and has been created in the laboratory - researchers managed it as long ago as 1989, using two electrified plates in a vacuum, and it has been achieved several times since. But it is far from clear that it would be possible to create it in the density required to curve spacetime enough for a wormhole.

"There is a term in physics, the 'weak energy condition', which is essentially a limit on the kind of matter than can exist in nature. Wormholes would need a material which violates that, and which is totally unlike anything we can make," says Pontzen. A further problem is that creating stuff that bends spacetime that much requires phenomenal amounts of energy; the sort of amount that could power a star.

Also, anything that curves spacetime that much is dangerous for anything you're putting through it. "Being near a black hole tears you apart," says Whyntie: the gravity at your feet is so much stronger than the gravity at your head that you are pulled into a long, thin line, a process that is worryingly described as "spaghettification".

"The trick is being able to use the wormhole without the thing you're sending being destroyed," he says. "But essentially, building a wormhole is just a question of nipping and tucking spacetime."

If, now that physicists have fobbed the problem off on to engineers, the engineers actually manage to make a wormhole, there will still be limits on what it can do. We won't be popping off in wormholes to any distant galaxy we happen to choose, says Pontzen.

"Both ends of your wormhole are in the same place when it's made, so you need to carry one end of it to the place you want to travel to," he says. What that means, in real terms, is that the universe's speed limit is still firmly in place - the first time you travel somewhere. But if you bring one end of a wormhole with you, you can then pop back through the hole to where you came from.

So it's an interstellar transport network, a sort of Tube system: "You couldn't use it for exploration, for travelling to new places," says Pontzen, "unless you stumbled across one that had been built by a previous alien civilisation." That, by the way, seems to be what happens in Interstellar.

Once you've got one in place, though, the things you can do with it are seriously strange. Remember that if you travel fast enough, time slows down for you: so if you went to, say, Proxima Centauri at near-lightspeed, although more than four years would pass for people back on Earth, you might experience the journey as just a few days. And this, says Pontzen, means time travel.

"If I took one end of a wormhole with me, it would experience time the same way; while one end of the wormhole would have experienced decades, the other end would have barely aged. So if I were to go back to Earth on my wormhole, I would travel 'back' to the Earth of years before, while someone going from Earth to my end would leap forward."

According to Thorne, this doesn't bring up the "go back in time and kill your father" paradox problem of time travel, partly because, as Whyntie points out, you can't go further back in time than when the wormhole was first created.

One of the characters in Interstellar discusses this in passing. There is also an intriguing line in the trailer, which seems to say that "one hour [through the wormhole] will be seven years back on Earth". Which doesn't sound like how it works, says Pontzen. "It doesn't sounds right, but without knowing more, I wouldn't want to say that Kip Thorne was wrong."